docs: def source_hashed(source_filename, prepared_options, thumbnail_extension,
                    **kwargs):
      """
      Generate a thumbnail filename of the source filename and options separately
      hashed, along with the size.
  
      The format of the filename is a 12 character base64 sha1 hash of the source
      filename, the size surrounded by underscores, and an 8 character options
      base64 sha1 hash of the thumbnail options. For example:
      ``1xedFtqllFo9_100x100_QHCa6G1l.jpg``.
      """
      source_sha = hashlib.sha1(source_filename.encode('utf-8')).digest()
      source_hash = base64.urlsafe_b64encode(source_sha[:9]).decode('utf-8')
      parts = ':'.join(prepared_options[1:])
      parts_sha = hashlib.sha1(parts.encode('utf-8')).digest()
      options_hash = base64.urlsafe_b64encode(parts_sha[:6]).decode('utf-8')
      return '%s_%s_%s.%s' % (
          source_hash, prepared_options[0], options_hash, thumbnail_extension)